Process for obtaining spatially-organised nanostructures on thin films

ABSTRACT

A process for forming nanostructures comprising the step of applying on localised regions of a smooth thin film of bistable or multistable molecules an external perturbation with preset magnitude thereby said film undergoes a collective morphological transformation and nanostructures are formed by selforganisation of said molecules, said nanostructures having preset number, size, interspacing and shape. The nanostructures can be used as storage medium in storage devices.

The present invention regards a process for forming nanostructures thatare spatially organised and with controlled size. The process isscalable, and is particularly useful for writing and storing informationat ultra-high density and large writing rate by forming of strings ofnanometer-sized dots or strips starting from a smooth organic thin film.

BACKGROUND OF THE INVENTION

Innovative technologies for information storage are aiming to reach theterabit limit, viz. to write more than a trillion bits on an inchsquared. The magnetic hard disk drive (HDD) is today's most widely usedmass data storage technique. Densest magnetic storage has beendemonstrated recently to have reached an areal density of 100 Gbits persquare inch (Gbpsi), using perpendicular recording technology. [1]State-of-the-art industrial production fabricates devices with arealdensity on the order of 50 Gbpsi. Although the annual rate of increasein the areal density of HDD is 60-100%, it is believed, although notproven yet, that the magnetic technology should break down beyond the200 Gbpsi limit because of uncertainty in the read/write areal densitydue to superparamagnetic current effects.

Alternative techniques for mass data storage [2,3,4] have been pursued,whose potential is to write information at terabit density, and with apower dissipation comparable to magnetic storage writing. Scanning probemicroscopies (SPM) have been demonstrated already more than a decade agoas useful writing/reading tools. [5] For instance, bits can berepresented in the form of topographic indentations or protrusions on aflat surface. The unparalleled resolution, both horizontal and vertical,allows SPM to write 1 bit per square nm, which implies an areal densityof 600 Terabit per square inch.

This density is however accessible only on perfect crystal surfaces inultra-high vacuum, which are of no straightforward technological use.Moreover, a single probe SPM is excessively slow, with best data ratesdemonstrated of 100 kbit/s in writing, and 1-10 Mbit/s for reading.[6,7,8]

A parallel data storage system based on SPM has been developed in thelast decade by researchers at IBM Zürich [9,10]. It is athermomechanical process operated by an array of cantilevers, termed“millipede”, each of them carrying an independent resistive probe [11].The resistor can be heated upon appyling a suitable voltage, and anindividual “bit” can be written as an indentation of the hot tip in athermoplastic polymeric film. The read out process is based on measuringthe heat loss from the tip to the substrate, which is lesser when thetip is above an indentation. Local heating erases the indentation, sothe technology is re-writable. By rastering the polymer film below thearray of cantilevers, information can be written and read on a largearea, at a data rate which is proportional to the number of cantilevers,but is inversely proportional to indentation time and limited byrastering speed. On these basis, the millipede system could support datarates as high as 1-2-Megabits per second. Power consumption is small(100 mW), due to the small displacements of the storage medium withrespect to the millipede. This is compatible with flash memorytechnology and considerably below magnetic recording. A millipede with1024 cantilevers was fabricated, and a terabit density demonstrated.[12] The millipede technology has also some drawbacks: i) each tip canonly write bits one by one; 2) a percentage of non-working levers leavesun-written areas; iii) the film must be sufficiently smooth to let thepassive system of cantilevers to operate without individual adjustementsof the tips above the surface.

Other processes based on SPM allow one to write information in the formof dots, rather than indentations. Among the highest areal densitiesachieved, local oxidation of a substrate by scanning force microscopy[13] has demonstrated the highest areal density with dots 1 nm high,20-40 nm wide and less than 20 nm apart. However, the dots cannot beerased and re-writing is not possible. This approach can be upscaled toparalell writing by using a multiple source of conductive protrusions,either a “millipede” with conductive tips, or a metallic or metal coatedstamp. [14,15]

Novel strategies for information storage technology rely uponmultistability. Multistable systems can be controllably switched betweendifferent configurations of comparable free energy. Multistability isintrinsically present in molecular and supramolecular systems through avariety of properties (conformations, co-conformations, redox and spinstates, shape and dimensionality) which can be influenced by externalstimuli (such as mechanical, electric, thermal, light). However, most ofthese changes manifest themselves only over length scales of, at best, afew molecules and in solution.

SUMMARY OF THE INVENTION

An aim of the present invention is to provide a process for obtainingnanopatterning of soft matter and thin films, for information storage atultra-high density, and for other applications wherespatially-controlled nanostructure are useful.

Another aim of the present invention is to provide a process that makespossible to generate, simultaneously, an arbitrary number of dots, from1 to as many dots are desired, organised into strings along well-definedlines on a thin film deposited on a substrate, the dots having the samesize and being periodically spaced.

Still another aim of the present invention is that of providing aprocess for forming nanostructures that is scalable and allows to obtainhigh areal density of dots.

A further aim of the present invention is to provide a process thatallows to controllably switch multistable systems between differentconfigurations of comparable free energy, particularly allowing toaffect a variety of properties of a molecular and supramolecular system,such as conformation, co-conformations, redox and spin states, shape anddimensionality, by changes occuring in the solid state, and their effecti being amplified across multiple length scales in terms of amorphological change that allows one to read and address them.

Another aim of the present invention is to provide a process useful forwriting and storing information at ultra-high density and large writingrate by forming of strings of nanometer-sized dots or strips startingfrom a smooth organic thin film.

These and other aims of the present invention, that are detailed in thefollowing description, are reached by a process for formingnanostructures comprising the step of applying on localised regions of asmooth thin film of bistable or multistable molecules an externalperturbation with preset energy thereby said film undergoes a collectivemorphological transformation and nanostructures are formed byselforganisation of said molecules, said nanostructures having presetnumber, size, interspacing and shape.

The formed nanostructures can be in the form of dots, when said regionsare one-dimensional and said nanostructures are in the form of strips,when said regions are two-dimensional.

The dots can be formed with a density, inter-dot distance or pitch andsize controlled by presetting the thickness of said thin film.

The dots can be formed in a number controlled by presetting a length ofsaid regions.

The nanostructures can also be organised in the form of arrays ofnanostructures.

The dots can formed with the process of the present invention with arealdensities of 1-1000 Gbpsi and can be used to code information.

The perturbation used in the present invention can be selected from amechanical perturbation, a thermal perturbation, a thermo-mechanicalperturbation, an electrical perturbation, a magnetic perturbation, aperturbation made with light or combinations thereof.

Moreover, the perturbation used in the present invention can be appliedwith a scanning probe microscope (SPM), or with mechanical devices,millipedes or actuators able to produce multiple local perturbations.

The perturbation can also be applied with a rigid stamp or with aflexible stamp with which a load force is applied on said film regions.The load force used can be preset, depending on the nature of the film,in the range of 0.1 to 100 kg/cm2.

The perturbation can also be constituted by a monochromatic light shonethrough the objective of an optical microscope, including a scanningconfocal or scanning near-field optical microscopes. This can also bedone with photolithography setups.

The morphological transformation of the thin film involves a variety ofphenomena that introduce spatial correlations and, at the same time,remain localised to the region of the thin film where the perturbationacts. Examples are wetting/dewetting transitions by nucleation of holesor droplets, spinodal dewetting, spinodal decomposition, crystallisationor formation of intermediate metastable structures.

The molecules of the thin film used in the present invention can beselected from the group consisting of rotaxanes also endowed withoptically or electrically active groups.

The molecules can also be selected from the class of catenanes and frommolecules having an isomerizable double bond, particularly moleculescontaining a linear C═C bond exhibiting a cis-trans isomerisation, azo ediazo groups.

The molecules can further be selected from molecular motors andactuators and biological motors, particularly actine, miosine,oligopeptides, DNA, RNA and oligonucleotides.

The thin film used in the present invention can be deposited on asubstrate or grown on a substrate form solution, or from vapour phase,or from reactive precursors, or by sublimation.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 shows examples of rotaxanes used to demonstrate the process.

FIG. 2 shows a schematic illustration of the dynamical process of thepresent invention

FIG. 3 shows examples of dot writing on the surface of thin films ofrotaxane 1 in FIG. 1, on graphite substrate.

FIG. 3 a shows an array of nanostructures.

FIG. 3 b shows parallel lines of different fabricated length thatyielded different number of dots.

FIG. 3 c shows the application of the process on a 30×30 squaremicrometer, with 31 lines consisting of 45 dots each.

FIG. 3 d shows an application of the process for dot writing to thestorage of hexadecimal numbers in the form of parallel strings.

FIG. 4 shows the linear relationship between the number of dots and thescan length for a film of preset thickness.

FIG. 5 shows the value of inter-dot spacing (left axis), the dotdiameter and height (right axis) versus film thickness.

FIG. 6 shows a schematic illustration of the static process of thepresent invention.

FIG. 7 shows an ATM image result of the morphological re-organizationfollowing the static process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention regards a new process that allows the formation ofnanostructures by self-organization of bistable molecules, starting froman initially smooth and featureless thin film. The nanostructures areformed only when a local external perturbation, of well definedintensity, is applied. The nanostructures can be controlled and presetin number, size, distance, form. Such nanostructures with the saidcharacteristics can be used to store information or other usages thatwill become clear in the following.

Storage information media can be obtained with the process of thepresent invention by obtaining of a thin film of bistable molecules,preferably taken from the class of rotaxanes. Rotaxanes consist of amolecular macrocycle locked around a molecular thread, that has twobulky groups, the stoppers, at both ends to prevent the macrocycle fromslipping off the thread.

Examples of rotaxanes used to demonstrate the process are illustrated inthe scheme FIG. 1. Their synthesis has been described in severalpublications [16]. Several studies, mostly in solution, havedemonstrated multi-stability of different co-conformations (viz. theposition or orientation of the macrocycle with respect to the thread)for this class of molecules. The rotaxane architecture reminds then agear around a shaft, else a molecular abacus. This has inspired thepossibility of using them as molecular machines and logical switches forultra-small device applications. By controlling rotaxane switching inthe solid state would be a powerful information storage method, providedone can amplify its effects to larger length scales. Other moleculeswith similar characteristics are catenanes formed by two interlockedmacrocycles.

Other molecules that exhibit a similar behaviour are taken from theclass of calixarenes and molecular cages; proteins and oligopeptides,such as miosine and actine; DNA, RNA, and oligonucleotides; linearsystems containing an isomerizable double carbon bond, else an azo- ordiazo-group, like stilbene, azostilbene, and diazostilbene.

In one embodiment, the present invention concerns a process that allowsto form nanostructures on a thin film made of rotaxanes. An importantfeature of the process is that nanostructures are formed collectively,and not individually, because of a spatially-controlledself-organisation process, where the molecules themselves move to formthe nanostructures of preset size and shape, and at the proper distanceswith preset regularity, only when a local perturbation of well definedmagnitude is applied. The perturbation does not control size anddistances, that are preset by the film thickness only.

If the perturbation acts on a rectangular area of the thin film, thenregularly spaced, equal parallel lines are obtained. If the perturbationacts along a line, nano-sized dots are formed along the same line. Thus,similarly to what is normally done in CD or DVD architectures, whereinformation is coded as alveoli along “lines” in the different sectors,the dots can be used as “bits” to store information on the thin film. Asan example, a 3-nm thick film allows information storage with an arealdensity close to 100 Gbpsi. We demonstrate that is possible to triggerthis transformation mechanically, using either a local probe, viz. thetip of an atomic force microscope (AFM), or, alternatively the forceapplied by linear protrusions of a stamp. It is important to remark thatall nanostructures are formed simultaneously by the collectivetransformation in the region that is mechanically perturbed: this iscompletely different from all other local-probe based lithographies,where the writing of the dots or indentations is made one-by-one.

The local perturbation provides the necessary energy to undergo themorphological transformation only in the region where the perturbationacts, leaving the regions outside completely unchanged. The size andspacing of the nanostructures depend on the film thickness, and hence isindependent to a large extent on the source of the perturbation. Thesteady-state of the transformation is reached independently on thedetailed parameters of the applied perturbation, apart for the totalpower dissipated during the mechanical interaction. Although thelimitations in data rate of the process demonstrated with the AFM tipare typical of SPM-based lithography, the writing can be upscaled toparallel approaches where a millipede or, more simply, a stamp can beused as a multiple sources of mechanical perturbation.

We demonstrate the scalability of the process by using a stamp whosemotif consists of a series of parallel lines.

The writing process is scalable in terms of velocity, of area and datathroughput, and density. The dots can be erased by heating the wholesubstrate, and, presumably, by applying a local heating using resistiveprotrusions. Thus, the process is viable for writing erasableinformation, else in non-volatile memories, or in recordable disks. Wefinally add that this process is not exclusive to the family ofrotaxanes or catenanes, but can be engineered by matching severalconditions of thin-film/substrate surface energy and surface diffusivityof the adsorbate the thin film is made of.

This process can be used to write non-volatile memories or recordabledisks with a density at least 10 times larger than the state-of-the artDVD technology. Other strength of our invention is the low-cost ofrotaxanes and molecular materials, the simplicity of the process of filmcasting and writing, and the scalability of the density, that requiresfilms progressively thinner.

Thus, the invention complies to many criteria of sustainability.

In an embodiment, the present invention involves a smooth, featurelessthin film of rotaxanes deposited or grown onto substrate. A localperturbation, mechanical in nature, is applied. The effect of suchperturbation is to provide energy onto spatially defined regions of thethin film. We take, as an example, the case of a mechanical perturbationthat is applied locally using the tip of an atomic force microscope(AFM). The perturbation consists in scanning the tip in contact with thethin film with a preset load force several times. Below a thresholdvalue of the force, the mechanical perturbation has no apparent effecton the region where it acts.

Above the threshold, the film is irreversibly damaged by theperturbation, as the tip scratches and plows the film with massivedisplacement of material. At the threshold, or around it in a very smallrange, the perturbation results in a morphological change in theperturbed region, from a smooth film to a structured film withnanostructures of defined size and characteristic distance. When theperturbation acts on a two-dimensional area, viz the ti is scanning onthe xy plane, the nanostructures consist of parallel stripes or strings,with the same width, length, size, and orientation.

Conversely, when the perturbation acts along a line, viz. the tip scansalong a single line, the morphology change consists in the appearance ofa string of dots. The evolution of the process can be visualised on anoscilloscope, by monitoring the time-evolution of the error signal, elsethe topography signal, as the tip scans. It can be observed that upon aninitial roughening along the scanned line, the dots emerge collectively,their size grows in time and their centers of mass arrange at the properdistances by self-organisation. No correlation either with the scan rateor other scan parameters was found. Once the steady-state is reached,continuing scanning does not yield any further change. The dots arecharacterised by defined size and spacing, that we demonstrate arecontrolled by the film thickness. In this respect, the process bearsresemblance to spinodal dewetting, although its time- and thicknessscaling differ. As the film thickness is preset on the region where theperturbation acts, the density is fixed, and the number of dots can begoverned or preset simply by choosing the length of the line where theperturbation acts.

The process is demonstrated on rotaxane thin films grown on a variety ofsubstrates: highly oriented pyrolitic graphite, silicon oxide bare orwith primers, glass, metallic thin films, polymers, mica, indium tinoxide (ITO), perovskite films (e.g. manganites). The film is depositedby drop or spin casting from a solution, and post-thermal annealed; itcan also be deposited by vapour deposition, by sublimation in vacuum orhigh-and ultra-high vacuum, CVD, plasma, self-assembling,Langmuir-Blodgett and Langmuir-Shaeffer techniques.

The process is demonstrated for thin films whose thickness was in the3-35 nm, but can still be valid for thicknesses of 1 monolayer orsub-monolayer, and thicknesses larger than 50 nm. No evidence of abreakdown of the phenomenon was ever observed. Suitable thin films werecasted from solution. They exhibit a smooth (<2 nm r.m.s. roughness),homogeneous coverage over a cm² area. In the case of rotaxanes, they arestable at ambient conditions in air, at room temperature, and ambientrelative humidity (≧50%) and upon exposure to light. Films show no signof degradation or change with respect to the as-grown film to AFMinvestigation after six months. Other deposition techniques orsubstrates can be used for the same purpose, provided the film qualityand thickness are similar.

DESCRIPTION OF THE DYNAMICAL PROCESS

The films can be imaged using Atomic Force Microscopy in contact modewithout any significant damage or wear for several times, if the setpoint force is kept below a threshold value, typically, of about 2 nN.This threshold value depends on the type of tip or protrusion utilised,and from the presence of contamination or overlayer. It also dependsupon the specific material forming the thin film. Therefore, thiscondition may be, in principle, changed from material to material.However, the threshold force can be phenomenologically discovered in afew attempts by systematically changing the setpoint force during scanand looking at instabilities appearing on the oscilloscope, as describedbefore.

Increase of the load force just above the threshold results in amechanical perturbation whose effect is localized to the contact area ofthe tip. When the tip is continuously scanned along a line, a string ofregularly spaced dots is formed. This process is schematicallyillustrated in FIG. 2.

The dots emerge upon repeating the linescan a number of times. Forsilicon nitride tips the number of scans to reach a steady-statearrangement of dots is between 4 and 20, depending on the scan rate(typically 1-2 Hz). Usually, at the same scan rate, longer lines requiremore scans in order to reach a steady string of dots. This implies thatthe total dwelling time of the perturbation, on the order of tens of μs,is the relevant parameter for the transformation to occur. Optimisedconditions allow us to obtain the structure even in one or two scans.Once the transformation is completed, further scans of the tip along theline do not result in any further change of the dots. Thephenomenological law that relates the number of scans, n, to thescanning speed v expressed in Hz reads: const=nv. This holds for apristine silicon nitride AFM tip when the load force has the value ofthe threshold force. By increasing the load force above the thresholdthe film is irreversibly damaged by massive displacement of thematerial.

In FIG. 3 some examples of dot writing on the surface of thin films madeof the rotaxane 1 in FIG. 1, on a graphite substrate.

FIG. 3 a shows an array of nanostructures 35-45 nm wide and arranged ona square lattice whose pitch is 140 nm along each line. This array wasfabricated on a 5 nm thick film by scanning near the threshold forcealong parallel lines. Scanning a series of lines results in a regulararray of dots of uniform width, height and pitch.

FIG. 3 b shows parallel lines of different fabricated length thatyielded different number of dots. This demonstrates that the number ofdots formed is proportional to the length of each linescan, so that anypredetermined number of dots can be reliably fabricated in the regionwhere the perturbation acts, once the thickness is fixed.

FIG. 3 c shows the application of the process on a 30×30 μm², with 31lines consisting of 45 dots each. The image also shows that the presenceof typical defects, such as terraces on the graphite plane, do nothinder or prevent the process. In the case shown here, the area issimply limited by the maximum scan range of our piezoelectric actuatorin our AFM. This shows the large-area potential of our invention thatwould allow, for instance, to use optical methods to locate the writtenregion, provided a proper chromophore is inserted in the rotaxanestructure.

FIG. 3 d shows the proof of concept of an application of the process ofdot writing to the storage of hexadecimal numbers in the form ofparallel strings, as in an abacus. The sequence: “e c 7 a 8” in thehexadecimal base corresponds to the number “968616”.

In FIG. 4 the linear relationship between the number of dots and thescan length, for a film of preset thickness, as from FIG. 3 b, is shown.The accuracy of the dot writing is better than 2%. Such value isestimated as follows:N_(dots)=aL, where a is the linear best fit of thewhole data set shown in the plot in FIG. 4, setting the offset to zero.The best fit slope with its estimated r.m.s. is equal to 0.453±0.006μm⁻¹ (for a 35 nm thick film). The value of the slope gives thecharacterictic length, else periodicity, and depends on the thickness ofthe film. Then, by using propagation of errors, the uncertainty on thenumber of structures is clculated as ΔN=ΔaL+aΔL. The second term isnegligible because of the ansatz of linear regression theory, and thebit uncertainty (18 in this case) of the D/A converter. Therefore,ΔN/N=Δa/a=1.32<2%.

In FIG. 5, we show the values of the inter-dot spacing (left axis), thedot diameter and height (right axis) versus film thickness. By varyingthe film thickness in the range between 3 and 35 nm, inter-dot distanceincreases from 100 to 500 nm, the dot full-width-half-maximum from 40 to250 nm, the dot height with respect to the smooth film from 1 to 20 nm,with a dispersion of 10-20%. The thicker the film the more spaced andlarger are the dots. The linear dependence on the scan length and onfilm thickness allows one to store information as strings of bits whosenumber is easily controlled.

Pitch P, diameter R and height h, with respect to the unperturbed film,can then be described solely as a function of film thickness, D: P=αD;R=βD; h=γD.

The thinnest films ('nm) yield dots whose apparent FWHM is 40 nm, theactual diameter is R=20 nm (from comparison with SEM images), h=1.5 nm,and P=110 nm. The estimated error on statistical samples of about 100dots is 10-20%. A value for P of 110 nm corresponds to an areal densitylarge than 60 Gbpsi. Extrapolation to 1 nm, viz. the thickness of amonolayer, allows us to estimate the minimum pitch to be 70-80 nm, andhence an areal density of 80-100 Gbpsi.

DESCRIPTION OF THE STATIC PROCESS

The mechanical perturbation that provides the energy needed for there-organisation of the film into dots can also be applied by means of astamp.

The process is schematically depicted in FIG. 6. The stamp is placed incontact with the thin film of rotaxanes, and then a suitable pressure(order of 1 kg/cm²) is applied. The process takes a few seconds, and theprinting time depends of the stamp's material, the size of the motifs,the film thickness, and of the particular rotaxane used. The writingvelocity with the static method depends also on the size of the stamp'sprotrusions. transformation is suitable for a new type of parallellithography based on a stamp as a multiple source of mechanicalperturbation. In FIG. 7, the AFM image shows the result of themorphological re-organisation following the static process. There, theprocess is applied to a thin film of rotaxane 1 deposited on graphite.The stamp consists of a sequence of parallel lines 400 nm wide and 100nm high, coated with a thin Au film. The applied pressure upon contactis 2.5 kg/cm2. The result is that, in correspondance of the protrusions,viz. the region where the intimate contact with the thin film isrealised, the film undergoes the morphological change into linearsequences of dots, and along all lines where the stamp was in contactwith the thin film.

Thus, by fabricating suitable stamps with protrusions of differentlength (e.g. a DVD), one can generate sequences with variable numbers ofdots along each protrusion's line, according to the individual lengths.This is an effective way to modulate the content of information across alarge area, and at the same time to fabricate smaller nanostructures,viz. the dots, using a featureless coarser instrument, viz. theprotrusion.

The application of the static method allows one to transfer in a fewseconds the entire information contained in a disk, e.g. a DVD, but witha resolution potentially on the order of 100 nm. With a one-squared inchstamp patterned with a density comparable to that of a DVD, viz. about 5Gbpsi, and a printing time of 5-10 seconds, writing speed for nonvolatile memories of 0.1-1 Gbit/s could be reached. This figure of meritis superior to that of any existing technology for non-volatile, nonmagnetic memories nowadays.

The re-organisation process has been observed and controlled, both inthe static and dynamical cases, with the rotaxanes only, but not withtheir individual components, macrocycle or thread. Also, a variety ofother molecules prepared in the form of a thin film have not exhibitedany of such re-organisation phenomena upon external stimuli. Theoccurrence of the collective re-organisation is the consequence of awetting/dewetting transistion that is triggered by the perturbation.Such a transition exhibits a phenomenology similar to spinodaldewetting, but follows different thickness-dependence, possibly due tothe viscoelastic characteristics of the material. Additionalexperimental evidence and modelling, both not reported here, showclearly that:

-   -   i) the rotaxane thin film is initially amorphous;    -   ii) a perturbation, that can be either mechanical or thermal,        provides the energy for molecules to form crystalline nuclei of        nanometer size. Such nuclei expose already the most stable        surface to the substrate. The detailed basal plane may depend on        the nature of the substrate, but is usually chosen among a group        of quasi-degenerate low-Miller indices planes of the rotaxane        crystals, in order to minimise tension and/or promote        commensurate growth where the substrate is crystalline such as        graphite. The nuclei formation is possibly eased by mobility of        rotaxanes or simply their internal degrees of freedom on the        surface. It is estimated that energy barries for rotation of        macrocycle in the solid state are smaller (−0.3 eV) than the        packing energy (2-3 eV), so circumrotation can occur without        massive displacement of molecules until the minimum energy with        the nearby molecules is achieved.    -   iii) the nuclei grow by ripening, incorporating smaller nearby        nuclei that form within a cut-off distance, forming large        crystallites. This process introduce spatial correlations that        eventually will manifest in the pitch;

as the crystallites reach a critical size, and they are approximately atthe same distance, the ripening process stops. The dots may grow even abit further by incorporation of individual rotaxane molecules diffusingalong the lines excited by the perturbation. The energy provided by theperturbation is not enough to complete the crystallisation process, andthe system reaches a steady state. The strings of dots represent anintermediate, metastable step towards the full crystallisation intolarge crystals with no long-range spatial correlations. In general, andbeyond rotaxanes, a material that can possesses the followingcharacteristics:

-   -   can be prepared in an initial amorphous or short-range ordered        film;    -   has limited adhesion with the substrate;    -   the components (atoms or molecules) have good surface        diffusivity even if their energy is below a temperature of        melting; they must not be mobile at room temperature;    -   it has a tendency to crystallise as energy is provided;        it may in principle be suitable to give rise to similar        controllable phenomena.

Examples may be for instance gold on quartz or glass with no adhesionlayer; conjugated oligomers on polar substrates.

The process of present invention allows to obtain a minimum demonstratedperiodicity of the dots of 100 nm, and a minimum apparent width 20-40nm.

The process of the present invention allows to the writing ofinformation shown on areas as large as tens thousands micrometers², dueexclusively to instrumental limits.

The process is also demonstrated to be scalable, and to be able toachieve, at least, areal densities in the range 100 GBits/in² (Gbpsi).

The present invention can be a viable route for storing non volatileinformation on an unexpensive soft medium, whose figures of merit arebetter than those of the current non-magnetic technology (CD, DVD, RVD),including technology not-yet on the market. The costs of the process andthe materials are extremely low, and the energy consumption during theprocess is very modest.

The field of application of the present invention is information storagemedia. The main sectors of application are a) backup systems, b)consumer electronics. As a comparison, the Blue Ray optical disk (likeDVD technology, but with a blue laser instead of a red one), developedby a consortium of nine large companies of consumer electronics, reachesa density of 15-20 Gbpsi. The main competitors of Blue Ray technologyare holographic techniques, heavily protected by patents owned by IBM.However, the latter have not been proved stable enough at roomtemperature (RT).

Other potential applications of the present invention are: flashmemories for mobile phones, disposable logic circuits and devices;identification media and markers for tagging and security; sensors ofmechanical force, or temperature, or chemicals; photo-sensitive thinfilms; adaptive, self-cleaning, biomicking, functional surfaces; masksfor patterning; gratings for photonics. Other sectors where theinvention will be potentially relevant are sensing and diagnostics;coatings and thin films; tribology; microelectronics processing;photonics.

The disclosure in Italian Patent Application No. BO2002A000759 fromwhich this application claims priority are incorporated herein byreference.

REFERENCES

-   1 Advanced Magnetic Storage and Medical Application    http://www.jst.go.jp/EN/chiiki/0715en/contents/2000_akita.htm-   2 Sandhu et al., U.S. Pat. No. 6,358,756 Mar. 19, 2002    “Self-aligned, magnetoresistive random-access memory (MRAM)    structure utilizing a spacer containment scheme”-   3 Zhou et al. U.S. Pat. No. 6,222,755; Apr. 24, 2001 “Solid state    holographic memory”-   4 Hua et al. U.S. Pat. No. 6,214,431 Apr. 10, 2001 “Optical data    storage materials for blue-light DVD-R”-   5 R. Garcia, M. Calleja, and H. Roher, J. Appl. Phys. 86, 1898    (1999).-   6 B. W. Chui, H. J. Mamin, B. D. Terris, T. D. Stowe, D. Rugar,    and T. W. Kenny, Appl. Phys. Lett. 69, 2767 (1996).-   7 B. W. Chui, T. D. Stowe, Y. S. Ju, K. E. Goodson, T. W.    Kenny, H. J. Mamin, B. D. Terris, R. P. Ried, and D. Rugar, J.    Microelectromech. Syst. 7, 69 (1998);-   8 P. Ried, H. J. Mamin, B. D. Terris, L. S. Fan, and D. Rugar, J.    Microelectromech. Syst. 6, 294 (1997).-   9 Drechsler U, Burer N, Despont M, et al. Microelectron eng. 67-8,    397-404, (2003)-   10 Binnig et al., U.S. Pat. No. 5,835,477 Nov. 10, 1998 “Mass    Storage Application of Local Probe Arrays”-   11 Eleftheriou E, Antonakopoulos T, Binnig G K, et al. IEEE T MAGN    39 (2): 938-945, (2003).-   12 Vettiger P, Cross G, Despont M, et al., IEEE T NANOTECHNOL 1 (1):    39-55, (2002).-   13 E. B. Cooper, S. R. Manalis, H. Fang, H. Dai, K. Matsumoto, S. C.    Minne, T. Hunt, and C. F. Quate, Appl. Phys. Lett. 75, 3566 (1999).-   14 Massimiliano Cavallini, Paolo Mei, Fabio Biscarini Ricardo    Garcia, “Parallel Writing by Local Oxidation Nanolithography with    sub-Micrometer Resolution” Applied Physics Letters 2003-   15 Cavallini, Biscarini, Mei and Garcia, Italian Patent    BO2003A000614: “Procedimento su larga area per l'ossidazione locale    del silicio e/o altri materiali mediante stampaggio su scale micro-e    nanometriche”-   16 Book “Mechanically-Linked Macromolecules” G J Clarkson & D A    Leigh in “Emerging Themes in Polymer Science”, Ed. A J Ryan, Royal    Society of Chemistry, Cambridge, pp 299-306 (2001)

1. A process for forming nanostructures comprising the step of applyingon localised regions of a smooth thin film of rotaxanes or catenanes anexternal mechanical perturbation with preset magnitude thereby said filmundergoes a collective morphological transformation and nanostructuresare formed by selforganisation of said molecules, said nanostructureshaving preset number, size, interspacing and shape.
 2. A processaccording to claim 1, wherein said nanostructures are in the form ofdots when said regions are one-dimensional and said nanostructures arein the form of strips when said regions are two-dimensional.
 3. Aprocess according to claim 2, wherein said dots are formed with adensity, inter-dot distance or pitch and size controlled by presetting athickness of said thin film.
 4. A process according to claim 2, whereinsaid dots are formed in a number controlled by presetting a length ofsaid regions.
 5. A process according to claim 2, wherein said dots areformed and used to code and store information with areal densities of1-1000 Gbpsi.
 6. A process according to claim 1, wherein thenanostructures are organised in the form of arrays of nanostructures. 7.A process according to claim 1, wherein said perturbation is appliedwith a scanning probe microscope (SPM).
 8. A process according to claim1, wherein the perturbation is applied with mechanical devices,millipedes or actuators able to produce multiple local perturbations. 9.A process according to claim 1, wherein said perturbation is appliedwith a rigid stamp or with a flexible stamp with which a load force isapplied on said film regions, said load force being in the range of 0.1to 100 kg/cm².
 10. A process according to claim 1, wherein saidmorphological transformation of said thin film is obtained bywetting/dewetting transition, dewetting introducing spatial correlation.11. A process according to claim 10, wherein said morphologicaltransformation of said film is obtained by spinodal dewetting,crystallisation or formation of intermediate metastable structures. 12.A process according to claim 1, wherein said molecules are selected fromthe group consisting of rotaxanes, rotaxanes terminated withoptically/electrically active groups and conjugated stoppers.
 13. Aprocess according to claim 12, wherein said rataxane is rotaxane
 3. 14.A process according to claim 1, wherein said molecules are selected fromthe class of catenanes.
 15. A process according to claim 1, wherein saidthin film is deposited on a substrate or is grown on a substrate formsolution, or from vapour phase, or from reactive precursors, or bysublimation.
 16. A process for forming nanostructures comprising thestep of applying on localised regions of a smooth thin film of rotaxanesor catenanes an external perturbation with preset magnitude thereby saidfilm undergoes a collective morphological transformation andnanostructures are formed by selforganisation of said molecules, saidnanostructures having preset number, size, interspacing and shape, theexternal perturbation being a mechanical perturbation applied with ascanning probe microscope (SPM) or with mechanical devices, millipedesor actuators able to produce multiple local perturbations.